Modern technology has produced a wide variety of electrical heating devices for use in cooking appliances. One class of electrical heating device which has been popular since the advent of readily available and economical electrical power has been the resistive electrical heating element.
Resistive electrical heating elements have been fabricated in a plurality of shapes over the years, in an attempt to achieve the ideal heating element, which is an element which will provide a flat surface having a uniform heat characteristic without exhibiting a gradient phenomenon or hot spots. In U.S. Pat. No. 4,233,497 to Herman H. Lowell, there is disclosed a heating element of a generally disc-shaped configuration in which the current flows radially, and wherein uniform heat generation over the surface of a heating element is achieved by providing the heating element with a shape wherein the thickness of the resistive material at a given radius from the center of the element varies inversely with the square of the ratio of the given radius to a reference radius at a reference point within the radial dimensions of the heating element. That is, the heating element is structured according to the equation ##EQU1## wherein T is the thickness of the resistive heating element, r is the radius at any point from the center of the element, and T.sub.a and r.sub.a are calculated at a reference point ("a"). The thickness of the electrically resistive heating element increases from the outer edges of the generally disc-shaped element towards the center in inverse proportion to the square of the ratio of the radius at any point to that of the radius at a reference point, the thickness value being varied while other parameters remain constant.
The ideal heating element has been approached by other prior art electrical resistive heating devices, but never actually accomplished in cooking appliances. For example, D. Harris in U.S. Pat. Nos. 3,351,742 and 3,383,497 on "electrical resistance heaters" teaches the concept in a graphite heating element of providing a graded thickness to the element, so that the electrical resistance characteristics of the heater will be uniform and a roughly uniform heat will be produced across the surface of the element. In an attempt to vary the parameters of the heating element, a plurality of holes are drilled through the heating element, to give an approximation of uniform heat distribution. By incorporating a plurality of holes in the heating element of Harris, heat flow variations occur which are experimentally chosen to cause the heating element to approach a constant surface temperature. Hole distribution is wholly empirical, an experimental approximation, and rests on no theoretical basis. Further, the heat transfer characteristics created by the holes drilled in the element result in undesired variations in the surface temperature, and thus the ultimate goal of a uniform temperature surface is approached, but not achieved.
Other U.S. Pat. Nos. 3,969,553 to Kondo et al, 3,870,776 to McMahon, and 3,833,386 to Wood et al show various shaped cermets, and combinations of ceramic bases and metal impregnation. These shaped cermets are without specific disclosure as to use as electrical resistance elements, or more specifically, electrical heating elements. None of these devices control variation of resistivity or angular coverage of the conductive material in combination with ceramic base.
Prior art resistance elements, deposited on the surface of a ceramic base, are shown by Bowman in U.S. Pat. No. 2,778,743 and Steigerwalt et al in U.S. Pat. No. 2,648,804. Conant et al in U.S. Pat. No. 2,698,990 discloses metal ceramics using designated particle sizes and proportions, which may be used as resistance elements. This general category of common resistance elements does not attempt to provide uniform heat distribution, as is desired in an electrical heating element.
The heating element of the present invention may utilize a sheath reinforcement structure to ensure mechanical integrity. Various types of reinforced structures are known, as shown by Brines in U.S. Pat. No. 1,960,328 and Decker in U.S. Pat. No. 4,156,997. The radial-membered structure of Brines is intended to be used in a building. The units of Decker et al are tension-compression equilibrium structures. Neither of these structures is known as an integral part of an electric heating element. The development of the present heating element has resulted in a further structural invention, that being the overcoming of stresses during operation via physical reinforcement of the electric heating element.